Molasses rust removal’s dark secret

I have been working on fixing up my grandpa’s old bench grinder for a while now. I’ll have more soon here on that project, but for now, I thought I’d share an update about something new I tested during the process: molasses rust removal.

Most of the grinder was rusty. Normally, a little rust is fine. But the base was just covered in it, so much so that the corrosion actually caused the power switch to fail.

I decided to start by targeting the rustiest parts. I soaked them in vinegar for five days. That wasn’t enough. Based on my previous experience with vinegar rust removal, I decided to stop there before I ruined anything. I hit the parts with a wire wheel to clean them further, but it took forever, and it still didn’t get all the rust.

Previously, a ton of commenters on my vinegar vs. rust video had mentioned that molasses rust removal was the way to go. So I thought, maybe this is a chance to try it out.

I decided to derust everything this time. I took apart the rest of grinder. Then I put the steel parts in a tote and covered them in a 1:9 mix of molasses to water, using jars of molasses from the grocery store. I’m glad I grabbed jars instead of bottles; they made it easier to refill and rinse to get out all the molasses during the measuring process.

I then brought the mix to sit down in the basement for a couple weeks.

Fast forward to 23 days later. I overshot my two-week deadline by a few days, but I lucked out. The parts didn’t seem to have dissolved.

Unfortunately, I discovered a different bit of news: The surface of the molasses was covered in mold. And it smelled terrible!

I donned a mask, put on some tall gloves and got to work. I scraped grime and rust residue off the steel parts with a brass bristle brush, which is too soft to hurt the metal. Then I rinsed everything off. Most parts got a coat of Prep & Etch to prepare them for painting and protect against further rust. Other parts that would be rubbing on each other for adjustment purposes would be getting no paint, so I protected those with oil instead.

After all that, the parts were mostly clean and rust free. But some of them still had rust spots. The base had one piece of rust so large that I could flake it off with a utility knife. That’s not unexpected for rust removal, but it’s important to know that molasses wasn’t flawless in this case. Maybe it would perform better with less water mixed in.

In review, there is a lot to like about molasses rust removal. It works. It’s forgiving. And it’s cheap.

But the mold part of the process was unpleasant, to say the least. I’m not at all tempted to try this again at this point because of that. Vinegar appeared to work just as well and was at least 79 person more pleasant to use.

The one exception here is if I needed to de-rust some giant part, like a car door or a motorcycle gas tank. I imagine it’d be a lot less costly to mix up 50 gallons of molasses solution than the other options, especially if you can find it for cheaper at a store that sells livestock feed. For example, Tractor Supply sells a 1 gallon jug of molasses that is half the cost per ounce if what you’ll find in a grocery store.